This could be the first major step we see in
shifting the industry from “consumer
electronics” to “consumer
photonics”.

At least that’s the opinion of Ritesh
Agarwal, associate professor at University Pennsylvania. He, along
with graduate student Brian Piccione of the Department of Materials
Science and Engineering in Penn’s School of Engineering
and Applied Science, and post-doctoral fellows Chang-Hee Cho and
Lambert van Vugt, also of the Materials Science Department, worked
on a study that concluded with not only their successfully
fashioning all-optical photonic switches out of cadmium sulfide
nanowires, but being able to combine them together to form a logic
gate, the fundamental component of information processing computer
chips.

Laser light being emitted from the end of
a cadmium sulfide nanowire.

Their breakthrough study was published in the
September issue of the journal Nature Nanotechnology.

Some back history

This has been a long time coming. The team built
this phase of their study upon earlier research which showed that
cadmium sulfide nanowires exhibit extremely strong light-matter
coupling. That means they’re efficient at manipulating
light, which is of significant importance in terms of the
development of nanoscale photonic circuits, a field in which
existing mechanisms for controlling the flow of light are way too
bulky and require way more energy than their electronic
analogs.

“The biggest challenge for photonic
structures on the nanoscale is getting the light in, manipulating
it once it’s there and then getting it out,”
Agarwal said. “Our major innovation was how we solved the
first problem, in that it allowed us to use the nanowires
themselves for an on-chip light source.”

What they did

Agarwal and his fellow researchers started
things off by precisely cutting a gap into a nanowire. Next, they
pumped energy into the first nanowire segment — to the
point that it began emitting laser light from its end as well as
through the gap.

Now, since the team was using a single nanowire,
the two segmented ends were an exact match with one another. This
allowed for the second segment to efficiently absorb and transmit
the light down its length.

“Once we have the light in the second
segment, we shine another light through the structure and turn off
what is being transported through that wire,” Agarwal
said. “That’s what makes it a
switch.”

The group measured the intensity of the light
coming out of the end of the second nanowire and showed that it was
enough to the point that their switch could effectively represent
the binary states used in logic devices. They then built a NAND
gate by combining two of these nanowire switches into a Y-shaped
configuration.

“Putting switches together lets you
make logic gates, and assembling logic gates allows you to do
computation,” Piccione said. “We used these
optical switches to construct a NAND gate, which is a fundamental
building block of modern computer processing.”

Quick refresher: a NAND gate stands for
“not and.” In terms of its computer processing
purposes, it returns a “0” output when all its
inputs are “1.” They are important because they
are “functionally complete”; that is, when
they’re put in the right sequence, they can do any kind
of logical operation, thereby forming the basis for general
computer processors.

“We see a future where
‘consumer electronics’ become
‘consumer photonics’,” Agarwal said.
“And this study shows that is possible.”
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